One of the difficulties involved with welding metal articles is that the process generates weld spatter which sticks to surfaces of various parts in the welding apparatus. Two parts particularly susceptible to deterioration by weld spatter are the contact tip and weld nozzle. When weld spatter gathers on an internal surface of the contact tip, it increases friction and reduces electrical contact with the wire electrode, thereby slowing operation. Spatter eventually causes the electrode discharge end of the contact tip to deteriorate and to have incidents (called "burn backs") wherein an electric arc extends back to the contact tip. These burn backs may interrupt operation by fusing the electrode wire with the contact tip and lead to frequent replacement of the contact tips, which adds expense by halting production and by increasing contact tip consumption.
Weld spatter also accumulates on interior and exterior surfaces of the weld nozzle. Accumulation of spattered metal on the interior surface reduces nozzle opening size, thereby perturbing gas flow over the weld arc. The spattered metal must be scraped frequently from the nozzle interior, also reducing efficiency of the welding operation. Eventually, spattered weld metal deteriorates the nozzle so severely that replacement is required.
In order to obviate the aforementioned difficulties, some ceramic coatings have been developed for welding apparatus in the prior art. However, such prior art coatings suffer from disadvantages which make them less than entirely suitable for their intended purpose.
Hein U.S. Pat. No. 3,430,837 discloses a welding tip including an inside coating that provides a non-stick surface. The coating comprises a high temperature ceramic or pyrolytic graphite or Teflon fluoropolymer.
Dietrick et al U.S. Pat. No. 4,450,341 claims a contact tip comprising a copper body and a wear-resistant member which may be formed of tool steel, metallic carbide alloys, or a ceramic composition.
Prunier U.S. Pat. No. 4,575,612 discloses a guide tube for an arc welding machine. The embodiment shown in FIG. 17 is a guide tube comprising a metal body having a longitudinal bore, the interior face of which is preferably provided with a protective layer of alumina. In a variation of Prunier's guide tube, this layer may be chromium dioxide.
Ceramic coatings utilized in the above-mentioned references simplify removal of weld spatter from metal parts. However, none of these coatings is sufficiently electrically conductive to allow utilization on internal surfaces of welding contact tips.
It is a principal objective of the present invention to provide a coating for welding apparatus comprising a film of a spatter-resistant, low friction, and electrically conductive substance.
A related objective of the present invention is to coat welding apparatus of the type described with a film that preferably comprises tungsten disulfide.
Additional objectives and advantages of the present invention will become apparent to persons skilled in the art from the following detailed description.